1. Field of the Invention
The present invention relates to an optical coupler and a method for producing the same. In particular, the present invention relates to an optical coupler in which a light introducing portion for introducing incident light into an optical waveguide is formed on a substrate so as to be monolithic with the optical waveguide.
2. Description of the Related Art
In recent years, technologies concerning optical waveguides have come to be rapidly and widely applied to optical fibers, optical integrated pickup devices, etc. The method for utilizing an optical waveguide falls roughly into two categories. One of the categories is a method in the optical communication field. According to this method, optical coupling is conducted by allowing the end face of a light source to be faced with that of an optical waveguide. In this optical coupling method, the mechanism for inputting light into an optical fiber is typically found in the art. The other category is a method for inputting light from outside into an optical waveguide through a light input portion such as a diffraction grating provided on the surface of an optical waveguide. In the case where an optical waveguide is used for optical pickup, light input means (coupler) such as a diffraction grating is provided on the surface of an optical waveguide.
According to the present invention, a coupler is provided on the surface of an optical waveguide, whereby light is input into the optical waveguide. Considering miniaturization of an optical pickup device by adopting the optical waveguide, this is an indispensable technique. A prism coupler and a grating coupler are often used as the coupler. In particular, a grating coupler is generally used so as to make a coupler portion integrated and thin.
FIG. 17 shows an application of a grating coupler in a conventional optical pickup device. As shown in this figure, an optical pickup device 100 includes a grating coupler 109, through which the optical pickup device 100 is optically coupled to a recording medium, i.e., an optical disk 110.
The grating coupler 109 functions so as to guide light diffracted by a diffraction grating provided on the surface of an optical waveguide 102 into the optical waveguide 102. The grating coupler 109 and the optical waveguide 102 constitute an optical waveguide element. In the example illustrated in FIG. 17, since the optical waveguide element is incorporated in the optical pickup device 100 for obtaining a magneto-optic signal, the grating coupler 109 of the optical waveguide element has a curved grating with a function of converging light onto the optical disk 110.
In the optical pickup device 100, laser light emitted by a light source (semiconductor laser) 101 is incident upon the three dimensional optical waveguide 102 formed on a substrate 107. The incident light propagates through the optical waveguide 102 during which the incident light is partially fed back by a grating 103 provided in the optical waveguide 102. This allows stabilized laser light to be output from the optical waveguide 102 to a slab optical waveguide 105 as a laser beam. This laser beam is collimated by a grating lens 108, propagates through a grating splitter 106, and is radiated to the optical disk 110 by the grating coupler 109.
The reflected light from the optical disk 110 is collimated by the grating coupler 109, split by the grating splitter 106, converged by the grating lens 108, and is incident upon detectors 104. The detectors 104 generate electric signals in accordance with the amount of the received light, respectively.
As described above, the grating coupler 109 of the optical pickup device 100 as shown in FIG. 17 converges light from the semiconductor laser 101 onto the optical disk 110, and inputs light reflected from the optical disk 110 into the optical waveguide element 102.
However, grating couplers have numerous problems. For example, the efficiency at which light incident upon a grating coupler is converted into light propagating through an optical waveguide (hereinafter, referred to as a coupling efficiency) depends upon the degree to which the condition of phase matching between light propagating through the optical waveguide and light diffracted by the diffraction grating is satisfied.
In the case where coupling is accomplished by the -1st order diffraction, the condition of phase matching is represented by the following equation: EQU N(.lambda.)=sin .theta..sub.i -.lambda./.LAMBDA.
where .lambda. is a wavelength of incident light in air; N is an effective refractive index (a value obtained by dividing a phase constant of propagating light by 2.pi./.lambda.); .theta..sub.i is an incident angle (an angle formed by an optical axis of incident light with respect to a normal of the surface of the optical waveguide); and .LAMBDA. is a grating pitch.
N(.lambda.) changes by about 10.sup.-4 /nm upon the change in the wavelength .lambda., while .lambda./.LAMBDA. changes by about 10.sup.-3 /nm upon the change in the wavelength .lambda. under the condition that a general value of .LAMBDA. is 1 .mu.m. The change in .lambda./.LAMBDA. is greater than that in N(.lambda.). Thus, the phase matching condition greatly changes with respect to the changes in wavelength of light incident upon an optical coupler.
In semiconductor lasers used as light sources in optical pickup devices, lasing wavelength is varied depending upon each laser and lasing wavelength changes due to the working environment temperature. Therefore, when light is coupled to the optical waveguide, a stable coupling efficiency cannot be obtained because of the changes in phase matching condition.
In order to solve the above-mentioned problem, in the conventional optical waveguide element shown in FIG. 17, laser light from the light source is partially fed back to the light source so as to suppress the changes in wavelength of output light from the light source, whereby a coupling efficiency is stabilized.
However, there is another problem causing the decrease in coupling efficiency in addition to the changes in wavelength. Diffraction is used for introducing light into the optical waveguide. This causes a loss of energy of light which is not diffracted at an angle appropriate for being coupled to the optical waveguide.
Furthermore, when a grating coupler and a portion for stabilizing the wavelength of light incident upon the grating coupler are provided on the identical substrate, the method for using the optical waveguide element is restricted.
Considering the above-mentioned problems, it is desired that a grating coupler be capable of reducing the changes in coupling efficiency due to the changes in wavelength and obtaining a high coupling efficiency. In this respect, prism couplers are outstanding for use in the optical waveguide element.
More specifically, according to the coupling principle of prism couplers, when light is incident upon the vicinity of an edge of a prism (the boundary between a portion where the prism is present and a portion where the prism is not present; i.e., the boundary between the prism and air), light, which is once introduced into an optical waveguide at a portion where the prism is present, can be prevented from going out of a portion where the prism is not present. For the above coupling principle of prism couplers to hold true, in the case where the base angle of the prism is substantially equal to an angle at which light is incident on the surface of the optical waveguide, a phase matching condition, which is represented by N(.lambda.)=n.sub.p (.lambda.) sin .theta..sub.i, where n.sub.p is a refractive index of the prism, is satisfied. The degrees of changes in N(.lambda.) and n.sub.p (.lambda.) in air with respect to the wavelength .lambda. of incident light are relatively close to each other. Therefore, the changes in the phase matching condition with respect to the changes in wavelength are smaller than those of grating couplers, and the tolerance of a coupling efficiency with respect to the changes in wavelength is greater than that of grating couplers.
FIG. 18 shows a conventional exemplary prism coupler disclosed in Japanese Laid-Open Patent Publication No. 3-87705. In this figure, a prism 115 is fixed onto an optical waveguide 111 with a dielectric adhesive 114 in a prism coupler 200. The optical waveguide 111 is provided on a substrate 118, with a first gap adjusting layer 112 and a second gap adjusting layer 113 layered onto the optical waveguide 111 in this order. The prism 115 is provided above an opening portion of the second gap adjusting layer 113. In the prism coupler 200, light 116 is incident upon the vicinity of the boundary between the dielectric adhesive 114 and the gap adjusting layer 113.
The coupling principle of the prism coupler 200 will now be described. The boundary between the dielectric adhesive 114 and the gap adjusting layer 113 in the prism coupler 200 corresponds to that between the above-mentioned prism and air. Light incident upon the vicinity of the boundary through the dielectric adhesive 114 is input into the surface of the gap adjusting layer 112 at an angle larger than a total reflection angle. However, the light passes through the gap adjusting layer 112 by a tunnel effect-like function to be incident upon the optical waveguide layer 111. The light incident entering the optical waveguide layer 111 is totally reflected from the boundary between the optical waveguide layer 111 and the substrate 118, travels to the gap adjusting layer 112, and is incident upon a thick portion of the first and second gap adjusting layers 112 and 113. At this time, the light is totally reflected toward the substrate 118 without being affected by a tunnel effect-like function. Due to the repetition of total reflection of light in the optical waveguide layer 111, light propagates through the optical waveguide layer 111.
According to this principle, optical coupling between the optical waveguide and other optical systems is realized by a tunnel effect-like function of light propagating through the thin dielectric layer on the surface of the optical waveguide. Therefore, light loss can be minimized and as a result, a coupling efficiency can be increased to about 80%.
As described above, prism couplers are effective. However, there is a limit to the size of a prism to be attached to the optical waveguide: the minimum size is a bottom area of about 1 mm.sup.2 and a height of about 1 mm. Thus, the size of an optical waveguide element which includes a prism and an optical waveguide is almost determined by the size of the prism. In other words, even in the case where incident light having a small beam diameter of, for example, 100 .mu.m or less is coupled, a substantially large prism is required.
Since prism couplers are thicker in their height direction, the integration of the prism couplers becomes lower compared with grating couplers. This means that the main characteristics of the optical waveguide element, such as thinness and smallness, cannot be exhibited.
Furthermore, it is more difficult to fix a smaller prism onto an optical waveguide. This results a unit value of one prism becoming more expensive.
In addition, prisms are required to be fixed one by one in the course of production of a coupler, causing the problem related to mass-production.